Typical and atypical gastrointestinal mast cells have been enumerated throughout the rat gastrointestinal system and correlated with tissue histamine. Typical mast cells degranulated after parenteral administration of compound 48/80, while atypical mast cells did not. These findings support the concept of heterogeneity in gastrointestinal mast cells. Acute administration of ovalbumin (OA) to OA-sensitized animals resulted in histamine release from gastrointestinal tissues, while chronic administration of OA resulted in an increase in atypical mast cell number. A sensitive enzyme-linked immuno-assay (ELISA) to measure total and specific IgE has been developed. This assay is rapid, uses widely available reagents, and may be applied to the isolation of specific food antigens.